The present invention relates generally to interference measurement in radiotelephone communication systems. More particularly, the present invention relates to estimation of interference on unused downlink channels prior to allocating a channel for a new call between a mobile station and a base station in a cellular telephone system.
The explosive growth in radio telecommunications requires continual improvement in the capacity of cellular systems. For example, one offshoot of this rapid growth is the gradual change internationally from analog cellular technology to digital cellular technology, which provides a five to ten fold increase in capacity. Improvements in technology alone, however, are insufficient to satisfy the rapid increase in demand. Thus, it is also necessary to improve the efficiency of existing cellular technology.
An important consideration in all radiocellular telephone systems is efficient use of the limited frequency spectrum allocated for cellular systems by the FCC. One method which has been adopted to maximize spectral efficiency is the frequency reuse scheme. In the frequency reuse scheme, the same channels can be simultaneously used to carry different calls in different areas, known as cells. One constraint on the frequency reuse scheme is that the cells must be spaced far enough apart that the interference between the two competing channels (called co-channel interference) is low enough to provide acceptable voice quality for users of the system. This interference characteristic conflicts with the desire to reuse the same channels as much as possible to maximize system capacity.
One measurement typically used to reflect signal quality in radiotelephone systems is the carrier-to-interference ratio (C/I). This ratio measures the strength of the carrier to the combined strength of any interfering signals. As discussed above, spectral efficiency, in the form of frequency reuse, pressures radiotelephone system designers to accept a relatively high level of co-channel interference to allow more frequency reuse.
Conventionally, a C/I of around 18 dB has been found to be an acceptable compromise between the competing goals of frequency efficiency and quality voice sound. Thus, many systems require that communication channels between a mobile and base station maintain a threshold C/I of about 18 dB. If the mobile unit either moves away from the base transmitter or moves to a location where interference is higher, such that the C/I drops below the threshold, the call will be handed off to another channel having sufficient quality.
In order to minimize handoffs which are required due to changes in quality within a cell, it is highly desirable to have knowledge of the interference on the channels available for communication before allocating any of these channels to a requesting mobile station.
One approach is to use a centralized decision system for channel assignment for each new call. For example, the received signal strength on control channels between each mobile station and each of a plurality of base stations in a cellular system can be monitored. The central processing system can correlate this information and make channel assignments dependent on the received signal strengths. One drawback to this centralized system is that it requires a great deal of measuring at the base stations as well as a lot of signalling between the base stations and the central system before a channel can be allocated.
This makes a decentralized approach much more feasible, i.e., a system wherein the allocation algorithm uses only data to which it has access without explicit reference to a multitude of other base stations. In its basic form the base station has access only to the signal strengths of the mobile station requesting a channel and to the interference in both up and down link on all channels that it can allocate.
However, time requirements on the call setup may not allow a solution where a mobile is ordered to measure the interference on down link channels during the call setup procedure. Additionally, some systems require that the base station transmit continuously on a carrier if the base station has a call on any of the time slots available on that carrier. In the latter systems it is not possible to measure the interference by using only a signal strength meter. In neither of these cases is it possible to anticipate the interference a particular mobile will receive prior to the allocation of a channel.